(Not to coding) When using SHA256 hash algorithm to find the nonce of the a sentence so that the final hash has x zeros in the beginning.

example: Today is a great day!########

######## is the nonce . The question is not about how to code, just want to know what the length of the "nonce" should be?

when generating the random number of ####### how should I know what is the right or min length of "nonce" ?